A long recognized need in rolling element ball bearing applications is a bearing that requires no lubrication, can operate at elevated temperatures, is lightweight and, has the ability to operate in a corrosive environment. All metal rolling element ball bearings require some form of lubrication due to the relative motion that is inherent between the balls and the balls raceways (inner and outer races). The relative motion is the result of two radii of different dimensions operating in contact with each other. The ball has a slightly smaller radius than the raceways thereby creating a line-contact arc curve region. The center point of the line-contact arc curve receives the greatest radial load between the ball and the raceways and has no relative motion during bearing operation. However, all points along the line-contact arc curve away from the center point are sliding with respect to the center point of the curve. The design intent of the line-contact arc curve is to displace the bearing loads imposed on the bearing members over an arc length sufficient to prevent permanent damage to the metallic bearing components. This life limitation can be in the form of brinelling (or indenting) of the metallic bearing surfaces, low cycle metal fatigue, or simply frictional wear of the bearing surfaces. Bearing life is extended through prudent design of the line-contact arc curve interface between the balls and the raceways such that the metallic material properties are not exceeded in normal bearing operation. In addition, lubricating fluids (oil, grease, or solid film lubricants) are used in the bearings to lower the friction between the balls and the races thereby extending bearing fatigue and wear life while reducing the bearing drive torque requirement.
Rolling element ball bearings containing metallic components (balls or raceways) are also limited to relatively low operating temperatures. In part, the temperature limitations are due to the degradation in strength of the metallic bearing steels with elevated temperature but also, temperature limitations are imposed because of the operating temperature limitations of the bearing lubricant. The use of dry film lubricants in place of oils and greases is an attempt to increase the operating temperature capability of some bearings without lubricating oil or grease temperature restrictions while minimizing the complexity of the lubrication system.
State-of-the art ceramic balls are being used in hybrid bearings (ceramic balls/metallic raceways) due to the lower dynamic friction coefficient between the ceramic balls and the metallic races versus that of metallic balls to metallic races. The high Young's Modulus and low density of ceramic balls also helps reduce ball deformation during high speed bearing operation for the former, and reduces ball skidding and outer race stresses for the latter. The weak link in hybrid bearings is still the metallic raceways due to metallic material limitations (i.e., compressive strength and hardness). Additionally, the contact region between the ceramic balls and metallic raceways still requires a line-contact arc curve and hence the requirement for some form of lubricant. The hybrid bearing lubrication requirements are less than those of an all-metallic bearing but nonetheless are required (in some amount) for normal bearing operation.
While most all-metallic bearings utilize the line-contact arc curve interface design, some metallic bearings utilize a concept known as a four-point contact bearing configuration. This concept creates point-contact between the ball and the raceway via a curved ball outer surface against a flat (or conical) raceway surface and therefore does not adequately distribute the imposed bearing loads along the contact surface of the metallic raceways. These point-contact (hertzian) loads result in very short bearing life and very low bearing load capacity and are generally used as stop-gap bearings only. Bearing steels have a relatively low compressive strength and hardness and will brinell quickly in point-load contact. The bearing surface of the balls and the raceways in a four-point contact bearing quickly degrade as brinelling occurs increasing interfacial friction and causing micro-welding and material pull-out between the metallic bearing components. The point contact zone immediately degrades to a relatively rough line-contact arc curve as the bearing surfaces rapidly degrade to failure. Continued bearing operation under these conditions always results in catastrophic bearing failure. The best metallic materials therefore will not support point load contact due to insufficient compressive strength and material hardness.